Re: [PATCH v2 3/5] MdeModulePkg/SmmLockBox: [CVE-2017-5753] Fix bounds check bypass

["Zeng, Star" <star.zeng@intel.com>]

Reviewed-by: Star Zeng <star.zeng@intel.com>

-----Original Message-----
From: Wu, Hao A 
Sent: Tuesday, September 25, 2018 2:13 PM
To: edk2-devel@lists.01.org
Cc: Wu, Hao A <hao.a.wu@intel.com>; Yao, Jiewen <jiewen.yao@intel.com>; Zeng, Star <star.zeng@intel.com>
Subject: [PATCH v2 3/5] MdeModulePkg/SmmLockBox: [CVE-2017-5753] Fix bounds check bypass

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1194

Speculative execution is used by processor to avoid having to wait for data to arrive from memory, or for previous operations to finish, the processor may speculate as to what will be executed.

If the speculation is incorrect, the speculatively executed instructions might leave hints such as which memory locations have been brought into cache. Malicious actors can use the bounds check bypass method (code gadgets with controlled external inputs) to infer data values that have been used in speculative operations to reveal secrets which should not otherwise be accessed.

This commit will focus on the SMI handler(s) registered within the SmmLockBox driver and insert AsmLfence API to mitigate the bounds check bypass issue.

For SMI handler SmmLockBoxHandler():

Under "case EFI_SMM_LOCK_BOX_COMMAND_SAVE:", the 'CommBuffer' (controlled external inputs) is passed to function SmmLockBoxSave().

'TempLockBoxParameterSave.Length' can be a potential cross boundary access of the 'CommBuffer' during speculative execution. This cross boundary access is later passed as parameter 'Length' into function SaveLockBox().

Within function SaveLockBox(), the value of 'Length' can be inferred by
code:
"CopyMem ((VOID *)(UINTN)SmramBuffer, (VOID *)(UINTN)Buffer, Length);".
One can observe which part of the content within 'Buffer' was brought into cache to possibly reveal the value of 'Length'.

Hence, this commit adds a AsmLfence() after the boundary/range checks of 'CommBuffer' to prevent the speculative execution.

And there is a similar case under "case EFI_SMM_LOCK_BOX_COMMAND_UPDATE:"
function SmmLockBoxUpdate() as well. This commits also handles it.

A more detailed explanation of the purpose of commit is under the 'Bounds check bypass mitigation' section of the below link:
https://software.intel.com/security-software-guidance/insights/host-firmware-speculative-execution-side-channel-mitigation

And the document at:
https://software.intel.com/security-software-guidance/api-app/sites/default/files/337879-analyzing-potential-bounds-Check-bypass-vulnerabilities.pdf

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
---
 MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.c b/MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.c
index 5a11743cb9..c1c9aa5663 100644
--- a/MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.c
+++ b/MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.c
@@ -76,6 +76,11 @@ SmmLockBoxSave (
     LockBoxParameterSave->Header.ReturnStatus = (UINT64)EFI_ACCESS_DENIED;
     return ;
   }
+  //
+  // The AsmLfence() call here is to ensure the above range check for 
+ the  // CommBuffer have been completed before calling into SaveLockBox().
+  //
+  AsmLfence ();
 
   //
   // Save data
@@ -160,6 +165,11 @@ SmmLockBoxUpdate (
     LockBoxParameterUpdate->Header.ReturnStatus = (UINT64)EFI_ACCESS_DENIED;
     return ;
   }
+  //
+  // The AsmLfence() call here is to ensure the above range check for 
+ the  // CommBuffer have been completed before calling into UpdateLockBox().
+  //
+  AsmLfence ();
 
   //
   // Update data
--
2.12.0.windows.1